Thermally regenerated low-reflectivity fiber Bragg gratings (RFBGs), as one mirror of a resonant cavity, have been introduced as linear-cavity fiber lasers combining with fiber saturable absorbers. The output of lasing presents an optical signal-to-noise ratio of 50 dB and temperature sensitivity coefficient of $15.36\mathrm{pm}/°\mathrm{C}$ for the heating process and $15.46\mathrm{pm}/°\mathrm{C}$ for the cooling process. The lasing wavelength variation and power fluctuation at 700°C are less than 0.02 nm and 0.21 dB, respectively. The RFBG-based fiber laser sensing has displayed good linearity for both the temperature rising and cooling processes, and favorable stability at high temperatures.